ISDN transmits data digitally and, as a result, is less susceptible to static and noise than analog transmissions are. Analog modem connections must dedicate some bandwidth to error correction and retransmission. This overhead reduces the actual throughput. In contrast, an ISDN line can dedicate all its bandwidth to data transmission.
Because ISDN is a digital service, it is sensitive to outside interference, and certain factors affect the quality of ISDN connections to the telephone network and the Internet. The computer connecting to the ISDN line must be within a given distance of the telephone company's central office equipment that serves the computer for the ISDN connection to work. The maximum distance allowed between the computer and the central telephone office is typically 18,000 feet. Additionally, ISDN connections can be adversely affected by telephone lines that are not wired using twisted-pair wiring.
Two types of ISDN service are currently available:
Basic Rate Interface (BRI)
Primary Rate Interface (PRI)
The most appropriate type of service for small Internet Information Server sites is ISDN-BRI. The most appropriate type of service for medium to large Internet Information Server sites is ISDN-PRI. The next two sections describe the two types of service in further detail.